Plate Tectonics Earth’s Operating System
G A Davie
RockandSky
Plate Tectonics
Earths Operating System
G A Davie
[email protected] www.rockandsky.net Copyright 2020
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Why Plate Tectonics?
Plate Tectonics is the lens through which earth scientists view their world. Life scientists also draw on the theory to explain and predict the evolution and distribution of species. The acceptance of the theory completely revolutionised the study of the Earth Sciences and provided a mechanism for explaining almost everything, which until then had not been available. 1965 was the year the world changed, for geologists at least.
Everything is a result of plate motion. The shape of the ocean basins, the high Alps and the high Himalayas, the volcanoes of the Japan and the Andes are due to plate tectonics. The two very destructive tsunamis of the last 20 years are a result of plate tectonics. Earthquakes in California or Nepal are the result of plate tectonics. Uplift, rejuvenation of rivers, and erosion are all the result of plate tectonics. Speciation of animals and humans alike is the result of plate tectonics. Mineral deposits, so important to our modern lifestyles, are the result of plate tectonics. Our very existence is a result of the sequestration of carbon via the Carbon Silicate Cycle – a feedback loop that keeps temperatures under control and by extension makes our planet habitable. Go search the Carbon Silicate Cycle on the Rock and Sky website to find out some more about this vital and fundamental process.
Geomorphology is fundamentally controlled by the underlying geology. Geology however is in thrall to plate tectonics and it is impossible to study physical geography without having a broad understanding of geology and plate tectonics. For example, huge granite inselbergs which poke their bald heads through ancient metamorphosed terranes create landscapes that are very different to those underlain by soft mudstones, which in turn are very different to landscapes underlain by hard, resistant sandstones. All landforms are to some extent controlled by the underlying rock types. Sure, there are other processes involved but fundamentally it boils down to geology.
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The study of physical geography should therefore fall under the umbrella of plate tectonics – it should be the first port of call in terms of framing the syllabus, for all the reasons given above.
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Chapter 1
The History
The story behind the development of the theory is a grand and fascinating tale. Some portions of the theory were worked out by independent scientists, some portions were worked out thanks to international collaboration, some scientists were ahead of the game, some were subject to the prejudices and chauvinistic attitudes of their day. Some weren’t even geologists. You can of course skip this chapter, but it is a wonderful story and what’s more, there are some fundamental concepts that this chapter introduces and frames beautifully and which will assist later on when we get to the nut and bolts of the theory. Read on to find out how big egos and big science eventually brought us to where we are now in terms of our understanding of the theory.
It was Abraham Ortelius (Figure 1.1), the leading map maker of the16th Century, who noticed the similarities between the shape of the African and South American coastlines.
Figure 1.1 Abraham Ortelius
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In 1855 Antonio Snyder Pellegrini pointed out the similarity between the coastlines of South America and Africa and ventured a reconstruction of the continents based on his observations, shown in Figure 1.2.
Figure 1.2 Pellegrini’s 1855 reconstruction
Later, HMS Challenger (Figure 1.3), after taking thousands of soundings of the depth of the oceans between 1872 to 1876, found that there was a prominent rise in the seafloor that ran the length of the Atlantic. The Germans confirmed these findings during the 1925-1927 Meteor Expedition, managing to trace the South Atlantic ridge through into the Indian Ocean. The Danes also carried out surveys of the ocean floor; and in fact there were host of nations running oceanographic expeditions at that time, including Sweden, Scotland, France, Holland, USA, Japan and Egypt.
Figure 1.3 HMS Challenger
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Italian geologist Roberto Mantovani (Figure 1.4) was also writing papers between 1889 and 1909, proposing an expanding Earth hypothesis to account for the shape of the ocean basins. American geologist Frank B Taylor, in 1908, also came up with an independent theory for the distribution of the continents but invoked the idea that the gravitational pull of the moon was dragging the continents towards the equator.
Figure 1.4 Roberto Mantovani
Then an upstart meteorologist put forward a more detailed and reasoned argument for continental drift in a book titled “The Origins of Continents and Oceans.” His name was Alfred Wegener (Figure 1.5), and he is now widely acknowledged as the father of the ‘continental drift’ theory. He became interested in the topic in 1911 when he accidently came across a scientific paper describing the distribution of identical plant and animal fossils across what we now know as Gondwana.
With his interest piqued, he carried out additional work on the subject, which ultimately led to him publishing his own scientific paper in 1912, and a book called “The Origin of Continents and Oceans” in 1915. It was initially published in German, but when the English translation came out in 1922 (Figure 1.6), the world was changed forever.
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Figure 1.5 Alfred Wegener
However, his ‘mad’ theory was not well received by the earth science community in Britain and America. Perhaps some of the hostility towards his ideas may have been partly due to anti-German feeling thanks to the long, bitter conflict of World War I. Also, as a meteorologist, Wegener was treading on the geologist’s toes – and besides, the idea did seem totally absurd - Wegener was not able to come up with a plausible mechanism to explain how continents could drift across the planet’s surface.
Figure 1.6 Wegener’s “The Origin of Continents and Oceans”
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But Wegener was an exceptionally clever man, who along with his father-in-law Wladimir Köppen, had authored the leading text book on meteorology. Nor was he a man given to sitting around inventing armchair theories, but was actively involved in leading expeditions to Greenland to collect meteorological data and measure the thickness of Arctic ice. He was also well versed in palaeobotany and palaeontology, and was acutely aware that Permian-aged Glossopteris coal fields were to be found in Africa, Australia, South America, Madagascar, India and Antarctica. Figure 1.7 shows a reconstruction of Glossopteris.
Figure 1.7 Glossopteris – the tree which makes up the Permian-aged coal fields of Gondwana.
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Figure 1.8 Lystrosaurus
Furthermore, a fossil dicynodont called Lystrosaurus (see Figure 1.8) had also been found in India, Antarctica and South Africa, and a fossil of a freshwater reptile called Mesosaurus (Figure 1.9) was discovered in southern Africa and Brazil. It would have been impossible for Mesosaurus to have made the journey across a salty Atlantic Ocean.
Figure 1.9 Mesosaurus skeleton
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At the time, most geologists believed in the notion of land bridges which allowed the movement of different species between the continents, but this land-bridge theory could not explain how a freshwater reptile could have migrated between South America and Africa. Finally, Cynognathus, a ‘dog jawed’ reptile was also found in South Africa, Argentina and Namibia. The distribution of this Gondwana flora and fauna is shown in Figure 1.10.
Figure 1.10 The distribution of flora and fauna across Gondwana
Figure 1.11 shows Dwyka Tillite, the Permian aged glacial deposits from South Africa.
Figure 1.11 Permian-aged glacial deposits from South Africa
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Based on his field observations, fossil evidence, discussions with other scientists, and his formidable powers of deduction, Wegener concluded that the continents had at one stage been joined together in a supercontinent, which he named Pangaea. Little did he realise that his big idea would meet with so much hostility.
For example, in Britain, a year after the first English translation of Wegener’s book, Philip Lake, who was then professor of Geography at Cambridge, had this to say at a meeting of the Royal Geographical Society: “He is not seeking the truth; he is advocating a cause, and is blind to every fact and argument that tells against it. It is easy to fit the pieces of a puzzle together if you distort their shape, but when you have done so, your success is no proof that you have placed them in their original positions. It is not even a proof that the pieces belong to the same puzzle or that all the pieces are present.” Ouch!
“Utter damned rot,” said William Scott, geology professor at Princeton, and Edward Berry, an American palaeobotanist, called Wegener’s theory “a selective search through the literature for corroborative evidence, ignoring most of the facts that are opposed to the idea, and ending in a state of auto-intoxication.” Whatever you might say about Berry, he certainly had a way with words.
Wegener tragically died in a Greenland blizzard in 1930, and his theory was orphaned, left to grow up in a hostile world without any real guardians to see her through to adulthood.
But there were some supporters of Wegener’s theory, all brave men in view of the hostile reception that his so-called crackpot theory had received. Their names shall be forever written in the Annals of Geological Fame – namely Reginald Daly, Alexander du Toit, and Arthur Holmes.
Reginald Daly (Figure 1.12) was a Canadian and a renowned field geologist who headed Harvard’s geology department, and had a passionate interest in basalt, claiming that “no rock type is more important to the Earth”. He recognized that oceanic crust comprised dense basaltic material, whilst continents were mostly made up of less dense granite. Quite early
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on, Daly agreed with continental drift and supported the idea with data he personally gathered around the world. He mischievously put the words E pur si muove! (“And yet, it moves!”) on the cover of his 1926 book, “Our Mobile Earth.”
Figure 1.12 Reginald Daly
Then there is Alex du Toit (Figure 1.13), a South African geologist, considered by some to be the greatest field geologist of them all due to the fact that he spent much of his life mapping the geology of South Africa, from the ancient granite basement, up through the entire sedimentary pile that makes up the Karoo Supergroup to the basaltic lavas that form the high plateaux of the Drakensberg.
Due to his unparalleled experience of Gondwana geology, Du Toit could not deny the evidence – the Late Carboniferous to early Permian-aged glacial deposits of the Dwyka Group, the Lystrosaurus, Mesosaurus and Cynognathus fossils that were found across what we now know as the Gondwana continents, and the presence of those pesky Permian-aged coal fields with their dominant Glossopteris flora.
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Figure 1.13 Alex du Toit
Seeking more evidence, Du Toit travelled to South America on a Carnegie Institution grant to inspect the geology on the other side of the water, which according to Wegener’s theory, would have been joined together some 200 million years before. In 1927 he published a paper on his South American geological travels called “A Geological Comparison of South America with South Africa”.
Ten years later he published his seminal book called “Our Wandering Continents” (Figure 1.14) which pulled all the evidence together to support the continental drift hypothesis. It was a brave move on his part, for as we have seen, our European and American geologists were not particularly open to such silly notions and had made their thoughts forcefully clear on the matter.
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Figure 1.14 Du Toit’s “Our Wandering Continents”
Two years later, World War II broke out, and perhaps the focus of the world, including that of the geologists, went elsewhere, allowing the theory to mature while battles were fought elsewhere.
Arthur Holmes (Figure 1.15) is the next player in this drama. He was a very influential and brilliant British geologist who developed a way to measure the age of the Earth using radioactive decay. He was the first to propose that Earth was over a billion years old, a number which flew in the face of the maximum date of 100 000 years put forward by the late, great Lord Kelvin. Holmes then figured out the mechanics of mantle convection and claimed that he had found the power source needed to make the continents drift. He also drew extensively on the writings of Wegener and Du Toit when presenting the evidence for continental drift in his book, “Principles of Physical Geology.”
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Figure 1.15 Arthur Holmes
This book was read by many generations of students, with the younger generation perhaps more open to new theories supporting the continental drift hypothesis. In Holmes’ book, one can see the reaching – some at the time would have said overreaching – for an answer to the questions that had puzzled earth scientists for decades, with the quest viewed partially through a foggy lens that Wegener had left behind for his disbelieving geologist counterparts. The final chapter of Holme’s 1944 edition is about the mobility of the Earth’s crust. It has the first ever drawing of mantle convection (Figure 1.16) and includes this line: “Currents flowing horizontally beneath the crust would inevitably carry the continents along with them.” Pure genius, but a very brave move back then.
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Figure 1.16 Holmes hypothetical mechanism for ‘engineering’ continental drift
World War II dragged on for six long years. At the end of it, everyone was exhausted, broke, and deeply affected by the waste of human life, resources and effort. But out of that madness, for the earth sciences at least, came some degree of consolation.
Figure 1.17 Captain Harry Hess
Captain Harry Hess (Figure 1.17), skipper of the USS Cape Johnson (Figure 1.18), had made voyages across the Pacific on submarine patrols during the war. His ship was equipped with sonar and an echo sounder, and having come from a geophysical background, Hess kept his
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equipment running even when they weren’t hunting for submarines. And thanks to that unplanned wartime surveying exercise he was able to collect ocean floor profiles across the northern Pacific, and that data was crucial in latter years at Princeton in supporting the continental drift hypothesis.
Figure 1.18 The USS Cape Johnson
Then in the 1950s, Ron Mason of Imperial College, London, working at the Scripps Institute of Oceanography, managed to convince the US Navy to allow him to tow a flux-gate magnetometer, developed by Victor Vacquier, behind one of their ships, to try and understand the geophysical nature of the ocean floor. The surveys showed that there were reversals of the magnetic signature of the basalts that made up the ocean floor. No one realised what these meant, and Mason went off to work on another project. He was to kick himself for that decision in years to come.
In 1962, Hess published a paper in which he coined the term ‘spreading centre.’ In it he presented his theory of sea floor spreading, the emplacement of basaltic lavas at the mid- oceanic ridges and mechanisms associated with this process. Wegener had been ahead of that one too mind you, observing in 1922 that ‘the Mid Atlantic Ridge Zone in which the floor of the Atlantic as it keeps spreading, is continuously torn open and making space for fresh, relatively fluid and hot Sima from depth.’
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Basalt, which is an extrusive igneous rock, i.e., a lava, contains a mineral called magnetite. When the lava is still in its fluid state the magnetite orientates itself towards the magnetic north pole. Once solidified, the orientation of the magnetite is locked in according to the position of the north pole at the time. Imagine a magnetised needle floating in a piece of cork in a bowl of water, which is then placed in the freezer. Come back several hours later and the water will be frozen, and the position of the floating needle in relation to the bowl, the freezer and of course magnetic north will be fixed in the ice.
Figure 1.19 The geophysical data from the eastern Pacific
The reversals of the magnetic signature of the ocean floor as recorded by Mason were first thought to be anomalous (Figure 1.19), until Drummond Matthews and Frederick Vine (Figure 1.21) of Cambridge University realised the significance of these ‘zebra patterns’ (Figure 1.20). In a 1963 paper they put forward the theory that new oceanic crust was being created at the mid oceanic ridges (which ran in a continuous range of mountains thousands of kilometres throughout all the ocean basins), with newer crust being located closest to the spreading centres.
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Figure 1.20 Magnetic reversal ‘zebra stripes’ from the eastern Pacific
When a magnetic reversal took place, this was recorded in new orientations of the magnetite in the newly-emplaced basaltic lavas along the spreading centre, with older reversals located further away from the centre. The real kicker is that the reversals were mirrored either side of the spreading centre. It was like having two conveyer belts, laid back to back, carrying newly added material away from the common centre at a constant speed.
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Figure 1.21 Frederick Vine and Drummond Matthews
Strangely enough, Lawrence Morley (Figure 1.22), a Canadian geologist, had written a paper a year earlier in which he had put forward exactly the same theory, but his paper was turned down by Nature, one of the world’s leading scientific journals, and so he had to publish in an obscure journal which no one bothered to read. Adding to the strangeness, Matthews and Vine also published their theory in Nature which was accepted, and they got the glory. It would appear that Morley had only submitted a letter without the backup data which is the reason for the it not being published. Geologists however are sporting people, and now credit is given in the name “Vine-Matthews-Morley hypothesis”. Ron Mason is now almost forgotten, which is a pity.
Figure 1.22 Lawrence Morley
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Earth scientists were now forced to re-assess of Wegener’s crackpot theory, for it was the only way to explain away the geophysical data. And to help them was Holmes’ work and the physical evidence in terms of fossils, flora and rock types supplied by Du Toit and Daly.
Then during the early 1960s Edward Bullard (Figure 1.23) from Cambridge used a computer to fit the African and South American continents together.
Figure 1.23 Edward Bullard
Instead of using the current shorelines, as other geophysicists had previously done, he used a depth of 914 meters (3000 ft) below sea level. This depth represents more accurately the edge of the continents.
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Figure 1.24 Bullard’s computer-generated fit of Africa and South America along the edge of the continental shelf. Blue indicates gaps and red overlap
By doing so he discovered a near-perfect fit of the African and South American shorelines (Figure 1.24). It turned out later that a similar reconstruction had been published thirty years earlier by a French geologist Boris Choubert but this work was printed in a Francophone journal of low international influence where it languished virtually unknown.
Girls, this one is for you. In 1952, Marie Tharp (Figure 1.25), a geologist and cartographer working under Bruce Heezen from Columbia University began to plot the sea floor profile data of the north Atlantic and found that there was a distinct V-shaped valley that was present in all of the profiles across the Mid Atlantic Ridge. She interpreted this as a rift valley, formed due to the emplacement of lava along the central axis of the ridge, thereby supporting the continental drift theory. Heezen dismissed her ideas, calling them ‘girl talk.’
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Another scientist, Howard Foster, was given the job of plotting the position of earthquake epicentres in the Atlantic and amazingly these epicentres lined up with the rift valley that Tharpe had plotted. She was now convinced that her theory was correct and went back to Heezen with the evidence – and give him his due, he accepted her hypothesis and became a plate tectonics convert. However, all the maps and scientific papers still went out with Heezen’s name on them, with no mention of Tharp.
Figure 1.25 Marie Tharp
She was ahead of the game too – if you remember Harry Hess had published his seminal paper in 1962 where he coined the term ‘sea floor spreading’ – but Tharp had beaten him to it by nearly a decade when she had put forward the idea that new crust was being minted in the rift valley that ran down the centre of the Mid Atlantic Ridge. Tharp is the genius responsible for the map of the ocean floors published in 1977 (Figure 1.26) by National Geographic and which we are now all familiar with, although it has been updated somewhat. In a male dominated world, she battled to find her space, but eventually as attitudes changed, she made her mark and was recognised and rewarded for her work.
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Figure 1.26 Marie Tharp’s map of the ocean floor
In 1965, a Canadian geophysicist John Tuzo-Wilson (Figure 1.27) came up with the idea of ‘plates’ to explain how the system worked – stating that the Earth’s crust was divided up into a number of plates that moved around on the planet’s surface, with new crust being formed and spreading centres and older crust being destroyed somewhere. Tuzo-Wilson also came up with the theory of transform faults which was fundamental in understanding how differential spreading rates along a mid-oceanic ridge re accommodated. We will look at this more closely when we get down to the nuts and bolts of the mechanisms.
Figure 1.27 John Tuzo-Wilson
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Additional geophysical work confirmed the location of the subduction zones along the destructive plate margins – most notably that of Hugo Benioff (Figure 1.28), who discovered a planar zone of seismicity which corresponded with the sinking slab in subduction zones – some of these earthquakes extending to depths of 670 km. Benioff had been studying earthquakes on the west coast of South America and the Kermadec-Tonga Trench which runs from New Zealand up through the Pacific to Samoa, including the islands of Tonga.
Figure 1.28 Hugo Benioff
As early as 1949 he had written a paper titled “Seismic Evidence for the Fault Origin of Oceanic Deeps” in which he presented a cross section showing the location and depth to the earthquake epicentres below oceanic trenches. The plot showed that there was very little seismic activity along the immediate axis of the trench, but moving towards the land in the case of South America, or the islands in the Pacific case, earthquake activity increased, along with the depth of the earthquakes. The plot is shown in Figure 1.29.
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Figure 1.29 Benioff’s plot of earthquake epicentres on the Tonga-Kermadec Foredeep
Kiyoo Wadati (Figure 1.30), a Japanese researcher, also discovered these zones in an independent study, but had written about them in Japanese, which made his discoveries inaccessible to English speaking geologists. These earthquake zones are now known as Wadati-Benioff Zones.
It was realised that those foredeeps which Benioff had discovered and which manifested themselves most spectacularly in the Pacific, were nothing less than the subduction zones where crust was being destroyed. No need for mad expanding-Earth theories to explain away the shape of the oceans. It all made sense now - new crust was created at spreading centres, with destruction taking place at Benioff’s subduction zones.
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Figure 1.30 Kiyoo Wadati
Once the theory of plate tectonics was accepted by the earth science community, geologists now saw the world through the lens that Wegener had left them, and what a wonderful view it was. And no one now had the academic authority to scoff at these ideas. In the US, which had been so anti-drift for years, geologists became fanatical converts, embracing the new theory with the same fervour they had used to try and debunk it 50 years before.
Things thereafter moved rapidly. The USA, having deep pockets, financed a drilling programme to explore the ocean floor, and a deep-sea research vessel called the Glomar Challenger was designed and built for oceanography and marine geology studies. The name was a nod to HMS Challenger which made the first soundings of an ocean trench - the Challenger Deep –- back in 1875. Little did the HMS Challenger crew realise that they were recording the deepest point on Earth and the site of a subduction zone. Starting in August 1968, the ship embarked on a 15-year-long scientific expedition, the Deep-Sea Drilling Program, criss-crossing the Mid-Atlantic Ridge between South America and Africa and drilling core samples at specific locations. Glomar Challenger is shown in Figure 1.31.
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Figure 1.31 The Glomar Challenger
As an aside, in 1960 Jacques Picard and Don Walsh journeyed to the bottom of the Marianas trench – the Challenger Deep – a descent of 10 911 metres - in the bathyscaphe Trieste (Figure 1.32), a trip fraught with perils possibly greater than the Apollo moon landings. They returned to tell the tale, although a window cracked during the descent thanks to the incredibly high pressures exerted by the surrounding sea water.
Figure 1.32 Bathyscaphe Trieste
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The age of the samples recovered by the Glomar Challenger was determined by palaeontological and isotopic dating studies, providing conclusive evidence for seafloor spreading and continental drift – now more correctly known as plate tectonics. No oceanic crust has been found to be older than 200 million years, which further supported the plate tectonic hypothesis in that anything older than that had been destroyed in subduction zones. This needs to be compared with continental crust, some of which has been dated at 4.1 billion years old.
Wegener’s orphan was now a beautiful princess with the world at her feet. By 1965 almost everyone was a plate tectonics convert, and scientists began to examine not only the oceans and the positions of the continents through Wegener’s lens, but the entire discipline. Suddenly everything was crystal clear and it was deemed to be exciting times for the earth sciences.
We now had a mechanism to explain the Alps, the Rockies, the Andes and the mightiest of them all, the Himalayas (Figure 1.33). Volcanoes along the US west coast, in Japan and Indonesia could be explained in the light of the new paradigm. The San Andreas fault, famous for demolishing San Francisco and Los Angeles on a regular basis, was seen to be a transform fault that had somehow manifested itself on dry land.
Figure 1.33 High Himalayas
The fauna, flora and identical rock types common to all the Gondwana continents could now be easily explained away without having to invoke crazy notions of land bridges and sunken continents. Plate tectonics explained the formation of mountains and oceans, mineral deposits and coal fields, earthquakes and tsunamis. It was so revolutionary that nothing in
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the earth sciences was untouched by it. Suddenly it all made sense – the Grand Unified Theory of Geology was here to stay, and I can just imagine Wegener saying: “gut gut, ja?”
Now that we have come to the end of that grand tale of big science, big battles and big egos, we can dive into the mechanisms of the theory.
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Chapter 2 The Structure of the Earth
We can’t talk about plate tectonics without a broad understanding of the structure of the Earth. If you have ever been down a mine you will know that temperatures increase with increasing depth. In some of the deepest gold mines on Earth, which extend to 4 km below the surface, temperatures reach 66o C, or 150o F. Imagine going down 10 times that depth where temperatures increase enough to melt rock. We have all seen volcanoes erupting and know intuitively that it is very hot at depth with temperatures of the inner core reaching 5 5000oC (9900oF). No one is able to travel to these depths, so Earth’s structure has been inferred based on geophysical data. Seismic waves caused by earthquakes have provided geophysicists with a way of ‘seeing’ into the interior of the Earth. Figure 2.1 shows how earthquakes provide data on the Earth’s structure.
Figure 2.1 The structure of the Earth based on seismological data
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These waves show that our planet is differentiated into distinct layers – more accurately, concentric shells - all of which have unique compositions and varying physical properties. Broadly speaking, these are the crust, the mantle and the core. The difference in density between these shells affect the velocity of seismic waves – and the boundaries between the layers are known as seismic discontinuities.
Figure 2.2 shows the composition of the Earth and, inset, the crust.
Figure 2.2 The composition of our planet – from crust to core
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The Crust Starting out with the familiar, let us examine the crust, for it is where we live. For all its apparent solidity, the crust is actually very thin and not that strong – much like that of a loaf - the outer thin brown layer holding together the lighter fluffier mass inside. Compared to the 6370 km from the surface to the core, the crust is only 6 km to 8 km thick beneath the oceans, and on average 35 km on the continents. In some instances, crustal thicknesses can reach 75 km, but that is a special case which we shall look at shortly.
Using another analogy, if Earth was a 1 metre diameter ball, the crust would be less than 2 mm in thickness. However, the crust plays an extraordinarily important role in geological processes near the Earth’s surface, and of course is extremely important in when it comes to supporting life.
Continental crust is made up of rocks rich in silicon, aluminium and oxygen with an average density between 2.6 g/cm3 and 2.7 g/cm3, whereas the oceanic crust is made up of rocks rich in magnesium, iron, silicon and oxygen, with a density of around 2.9 g/cm3 to 3.0 g/cm3. Therefore, because continental crust is less dense than oceanic crust, it will ‘float’ on top of the denser material. Oceanic crust almost exclusively comprises a rock called basalt whilst the continents comprise granites, gneisses, sandstones and mudstones along with a host of other rock types which we won’t get into just yet.
Another feature of oceanic crust is that it is no older than 200 million years. This might sound very old, but when you compare it to continental crust which is 4.1 billion years old in places, it is very young.
The Mantle Below the crust is the mantle, which extends to a depth of approximately 2900 km below the surface. The boundary between the crust and the mantle is called the Mohorovicic Discontinuity, named after a Croatian geophysicist called Andrija Mohorovičić who first recognised it. That is too much of a mouthful for even the geologists, so everyone just calls it the Moho. Simple enough.
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The top of the mantle is marked by the Moho, where there is a huge jump in the density between the crust and the mantle rocks. Density and pressure increase with depth, with an attendant increase in the velocity of the seismic waves. However, at a depth between 100 km and 200 km, the seismic waves slow down by around 7 percent, in what is called the Low Velocity Zone (LVZ). Here the rocks are close to their melting point, are therefore more plastic and less dense, with a corresponding decrease in the velocity of the waves.
The Core At a distance of 2900 km below Earth’s surface there is yet another major discontinuity, called the Gutenberg Discontinuity. This marks the boundary between the mantle and the core. The seismic data shows that the outer core comprises an exceptionally dense liquid which is thought to be mostly made up of nickel and iron. The inner core is solid and is believed to comprise an alloy of nickel and iron. The density of the core is around 10.8 g/cm3 compared to that of the mantle which ranges from 3.3 g/cm3 closest to the Earth’s surface to 5.5g/cm3 nearer to the core. Figure 2.3 is a recap of the composition of our Earth.
Figure 2.3 The composition of our planet – a recap
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Chapter 3
The Theory
We have already looked at the history of the development of the theory – how Alfred Wegener put forward the idea that the continents had all once been joined, based on the fact that there were Permian-aged coal fields on all of the Gondwana continents, that the same fossils were found between South America and Southern Africa and that the geology across the Atlantic matched.
Additional geological information had been provided by Alex du Toit who had travelled to South America on a Carnegie Institution grant to inspect the geology there, and confirmed that there were almost identical rock formations on both continents. We learned how the geological fraternity laughed at Wegener, and went out of their way to ridicule him. In 1930 Wegener tragically died (Figure 3.1) in a Greenland blizzard and never saw his theory go from being the laughing stock of the scientific world to its darling.
Figure 3.1 The site of Wegener’s death
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However once the theory had been accepted, earth scientists were given a new lens to view their world. They were now able to decipher the workings of the planet in new ways. It was a ‘eureka’ moment. Apologies to Leonard Cohen but:
“It goes like this, the fourth, the fifth, the minor fault, the major rift, the baffled geologist shouting ‘hallelujah!’”
Let us quickly recap. Alfred Wegener had written his book in which he speculated that "The Mid-Atlantic Ridge ... zone in which the floor of the Atlantic, keeps spreading, is continuously tearing open and making space for fresh, relatively fluid and hot sima [rising] from depth,"1 as a mechanism to explain how the continents were able to move.
Later Arthur Holmes had come up with the idea of convection currents carrying plates away from a central rift zone, publishing his ideas in his 1944 edition of his textbook “The Principles of Physical Geology.” His ideas proved to be prescient and ahead of the game in terms of arriving at a mechanism to account for plate movement.
Marie Tharpe had proven that there was a continuous rift valley running down the length of the Atlantic, and earthquake data had confirmed her theory.
Harry Hess had proposed the concept of sea floor spreading, and Drummond Matthews and Frederick Vine had confirmed his theory based on geophysical data.
John Tuzo-Wilson introduced us to the idea of tectonic plates and transform faults and Hugo Benioff had provided us with a mechanism for crustal destruction when he realised that the oceanic trenches were deep-seated fault zones.
1 Sima is an acronym for silicon and magnesium, which are the major components of the basalts which makes up the ocean floor. The term Sial as a counterpart to Sima, and is an acronym for silicon and aluminium – usually ascribed to continental crust. The terms have fallen into disuse in modern geology.
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The rusty signposts have brought us thus far, left by those pioneering map-makers, geologists and geophysicists who have gone before. It has been a grand journey but now we need to get down to the nuts and bolts of this amazing machine that drives our planet.
We now know that Earth’s crust is divided up into seven major plates. New crust is formed at mid-oceanic ridges, which leads to sea-floor spreading, much like two giant two conveyor belts running back-to-back, carrying freshly-minted crust away from the spreading centre. The magnetite minerals in the molten rock align themselves with the magnetic field prevailing at the time.
As the new crust cools, the magnetite locks in this orientation which provides a record of where the magnetic north was located at the time of cooling. This is an important point, because magnetic north flips its polarity periodically in what is called a magnetic reversal. Sometime in the future magnetic ‘north’ will be located near the current south pole. The magnetic signature of the rock can be measured using an instrument called a magnetometer and it was this signature which nailed down the theory back in the 1960s. The major tectonic plates are shown in Figure 3.2.
Figure 3.2 The major tectonic plates
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Ongoing magnetic reversals result in a striped ‘zebra’ pattern which ran parallel to the spreading centre, recording a positive orientation of the magnetite minerals when the magnetic north pole was located in the north, and negative signature when the magnetic north poled was located in the south. The real kicker here is that the reversals are perfectly mirrored on either side of the mid ocean ridge and perhaps even more amazing was that the reversals were the same age throughout all the oceans. Hard evidence indeed.
New crust is being minted all along the spreading centres of the oceanic plates and because the size of the Earth is essentially fixed, and if new crust is created at a spreading centre, we need a mechanism to account for all the extra material which is being created at the spreading centres.
The most obvious solution to the space problem is that crust has to be destroyed elsewhere. And that is essentially what happens. It turns out that the older oceanic crust is destroyed at the plate margins in what are called subduction zones. (There was a theory that supported an expanding earth hypothesis back in the day, but that theory has now been junked). The earthquakes that take place below the deep ocean trenches are due to the downward descent of the subducting plates as they are driven deep into the mantle.
National Geographic published a wonderful image of how it all works. It is a little exaggerated, and it is a little generalised, but it does a great job in illustrating the major components of the system. In the centre you can see the upwelling of mantle material, which erupts at the spreading centre as dykes and pillow basalts, and how the plates are carried left and right away from the centre, to be subducted at the plate boundaries. Huge convection currents operate in the crust, carrying the plates along on with them. The energy that drives this process is radioactive decay. The National Geographic image is shown in Figure 3.3.
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Figure 3.3 Upwelling at a spreading centre and subduction on the margins (National Geographic Society)
Another way of making space for new crust is by crustal thickening – which essentially means scrunching everything up. You can run this experiment on your kitchen table. A crisp white linen table cloth that is 2 metres long can be rucked up into a 1 m length, or even less, thereby shortening it. Where you have rucked it up you now have a stack of lovely, folded fabric lying in white piles above the table’s surface. And that is exactly what happens in fold mountains, where the rocks are folded and faulted into contorted, twisted masses that are a delight for some geologists and the nightmare of others. Geologists use the term ‘nappe’ to describe huge overturned folds, the term stemming from the French word for tablecloth which alludes to the folded white linen experiment just discussed. The Alps, and the granddaddy of them all, the Himalayas, provide fantastic evidence of the immense forces that play out in the Earth’s crust.
Plate Boundaries Plate boundaries are the most geologically active places on Earth. They are characterised by earthquakes, volcanoes and tsunamis, deep submarine trenches and towering mountain ranges. In short there is always some kind of topographic feature associated with them. We have already briefly talked of the Alps and the Himalayas, but the island arcs of Indonesia and
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Java (Figure 3.4), and the deep submarine trenches of the Pacific are also spectacular features – a kind of inverted Himalayas. Do you know that you could drop Mt Everest into the Marianas Trench, and there would still be nearly 2186 metres of water above the summit? No mistaking it, that is one deep gash in the Earth’s crust.
Figure 3.4 Mt Bromo and distant Mt Semeru, Java, Indonesia
The plate boundaries are broadly classified as: • Divergent boundaries • Convergent boundaries • Conservative or transform boundaries
The ongoing bump and grind of the plates over time leads to the development of dynamic systems where they meet. Internally the plates are mostly inactive – rigid and uniform (in the case of oceanic crust) plates. Where they meet, they bump, dive under one another, or pull apart. Or they may slide past one another. A bit like human behaviour in fact. Where there are bumps, there are earthquakes or ructions, where there is subduction there are eruptions and earthquakes, and where there is pulling apart there is space for new things – in this case lava. The sliding thing is the easy way out but also may come with its fair share of earthquakes.
Most plate boundaries occur in oceanic regions, and the ‘Ring of Fire’ is a fine example of this. Separation takes place along the mid-oceanic ridges, and convergence at the ocean trenches. Sliding takes place along transcurrent faults at major fracture zones.
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Divergent Plate Boundaries Constructive It is here that new crust is formed as the plates are carried away by convection currents, leading to rifting and magmatism at the mid oceanic ridges.
These mid-oceanic ridges, or spreading centres as they are also known, is where newly-minted oceanic crust is formed. The new crust is carried away from the central axis much like two conveyor belts working back to back. Whilst still molten, the magnetite minerals orientate themselves towards the magnetic north whereby their orientation is ‘frozen in’ when the rock passes through its Curie Point at around 583o C.
The components of the system are as follows. As the plates move apart, the crust and the underlying mantle thins due to the stretching and faulting that takes place. Hot mantle material rises beneath the lithosphere which causes an upward swell along the axis of the ridge – hence the term mid oceanic ridge. Faulting takes place as the plates pull apart to form a rift valley, with the crust stepping down to the central, active spreading centre which runs along the axis of the rift.
The rising mantle material partially melts due to a reduction in pressure, which leads to the formation of basaltic magma which accumulated in a chamber below the central rift. As the plates pull apart, this magma is injected upwards into the space left due to the rifting to form vertical dykes – a bit like toothpaste being squeezed into the gaps - resulting in the formation of new oceanic crust on the ridge axis as the plates ride away from the spreading centre. Some of the same magma finds its way onto the sea floor to form what are known as pillow lavas. Figure 3.5 shows a cross section through a mid-oceanic ridge and the components of the system.
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Figure 3.5 Cross-section through a mid-oceanic ridge
Pillow lavas are an interesting phenomenon. They form when lava comes into contact with seawater, causing the outer layer to solidify or ‘freeze’. However, the magma chamber still continues to pump hot molten rock into the pillow, which continues to expand until such time as the outer crust cools enough to prevent further expansion from taking place.
Once this point is reached, a new pillow forms below or adjacent to the ‘frozen’ pillow, and so the process continues until there is a pile of ‘pillows’ resting one on top of the other. These new pillows are then slowly carried away from the spreading centre by the ‘conveyor belt’ of the plate itself.
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Figure 3.6 Pillow lavas of the Omani Ophiolite – a geological classic. Note the man for scale.
They are of special importance to geologists because an outcrop of pillow basalts shows that these rocks were extruded underwater and which may in fact be ancient ocean floor. There are examples of remnant mid oceanic ridges with dykes, pillow lavas and magma chambers to be found exposed as rock outcrop in several places on Earth, thrust there by tectonic action. Figure 3.6 shows pillow lavas from the Omani Ophiolite, where there are spectacular exposures of pillows as well as exposures of the Moho (Figure 3.7) and underlying mantle. Ophiolites are found in numerous places on Earth including Italy, Cyprus, Scotland, South Africa, USA, India, Russia and New Zealand.
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Figure 3.7 Oceanic crust-mantle boundary exposed in Oman
It was Gustav Steinmann, a German geologist working in the Italian Apennines who first described pillow lavas, along with two other rock types called serpentinite and chert. These three rock types came to be known as the Steinmann Trinity. Serpentinite comprises basalt, and peridotite which have their origins as sea floor and mantle respectively but which have been altered and slightly metamorphosed due the origins reaction of the molten rock with the surrounding seawater. This turns them into a green, rock which feels ‘soapy’ to the touch. The final part of the ‘trinity’ were the cherts which were nothing less than the accumulation of millions of siliceous skeletons of Radiolaria – tiny creatures whose remains rained down from the ocean waters above and which continue to do so to this day. Figure 3.8 is a photomicrograph of Radiolaria from modern ocean floor sediments.
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Figure 3.8 Radiolaria which rain down onto the ocean floor and form chert deposits
Steinmann (Figure 3.9) understood, way before the theory of plate tectonics, that these three rock types which lay exposed under the Italian sun were nothing less than the remnants of ancient oceanic crust that had somehow been thrust up onto dry land.
Figure 3.9 Gustav Steinmann
At the time geosynclinal theory was being used to explain the formation of mountain ranges – a theory developed originally in the USA to explain the formation of the Appalachians, and which had found converts who needed a theory to explain Alpine geology. A satellite image of the tightly folded rocks of the Appalachians is shown in Figure 3.10.
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Figure 3.10 The Appalachians - an ancient mountain range ground down by 480 million years of erosion. Note the tightly folded structure
This theory has now been superceded by plate tectonics, but it was enormously influential for 100 years when it came to explaining the formation of mountain ranges.
We have got slightly ahead of ourselves here, so keep that thought about collision until we get to it in a later section.
Returning to the process that take place along a spreading centre, faulting parallel to the spreading axis allows seawater to penetrate deep into the oceanic crust, where it becomes superheated when it gets close the magma chamber. This superheated water then strips out minerals from the surrounding rock on its journey back towards the surface. Scientists were astonished to find that where these hot brines exit the rock, tall chimneys form, which vent plumes of black ‘smoke.’ It turns out that when the hot brines with their dissolved load of metal sulphides and other mineral compounds come into contact with the cold seawater, they
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precipitate to form tall chimneys which vent minerals into the surrounding ocean. These ‘black smokers’ have been filmed by the submersible Alvin of the Woods Hole Oceanographic Institution. Figure 3.11 shows emissions from a ‘black smoker’.
Figure 3.11 A black smoker
It is thought that many of our mineral deposits were formed in this way. Over time these portions of the ocean floor with their mineral deposits become caught up in the tectonic mill and find themselves thrust up onto continental areas to be discovered millions of years later by feisty geologists. Another example of why the study of plate tectonics is fundamentally important to our lives and our civilisation.
Scientists were also astonished to find a host of strange creatures called extremophiles living in these high temperature, high pressure environments of spreading centres. There are theories that life may have begun in these amazingly harsh, but strangely hospitable, places over 3.5 billion years ago. Extremophile barnacles are shown in Figure 3.12
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Figure 3.12 Extremophile tube-worm colony living in the hot water venting from faults within the mid oceanic ridge
Rift Valleys
Another type of divergent plate boundary can be found in continental areas, which can ultimately lead to the formation of new oceans. The East African Rift Valley is a fine example of this, where the crust has stretched and thinned due to the upwelling of hot material from the mantle in a very similar process to what is taking place at the spreading centres. Due to the stretching and tensional forces in the crust, normal faults form parallel to the axis of the rift which leads to downward faulting of blocks to form a rift valley.
Thinning of the crust due to tensional forces causes upwelling of mantle material which then leads to upward bulging of the crust along the axis of the rift. Faulting then takes place that steps down to the valley floor. Ongoing erosion of the valley sides fills the rift with debris, lakes form, and often there is volcanic activity due to the presence of hot shallow mantle material. All of this takes place on continental crust which is generally above sea level. However, if the rifting continues then a seaway may develop when the floor of the valley falls below sea level. Figure 3.13 shows how rifting starts and how with time an ocean may form.
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A) The crust and lithosphere are thinned by stretching and hot magma rises from below, leading to uplift. Faults along the axis of the uplift produces a rift valley with associated volcanism. The East African Rift Valley is an example of early rifting.
B) Over time, basaltic magma due to the volcanism is intruded into the space between the divergent continental blocks. Subsidence due to the density difference between ocean crust and continental crust allows for the formation of a seaway between the two continental segments. The Red Sea is at this stage of rifting.
C) Ongoing rifting leads to the development of a mid-oceanic ridge and a true ocean basin. The Atlantic Ocean is a good example.
Figure 3.13 The formation of a rift valley and the possible development of a new ocean basin.
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It is expected that the East African Rift Valley will in time become a seaway much like that of the Red Sea which has a divergent plate margin running along its central axis. This is how rifting starts, and the east coast of southern Africa and Antarctica would have been separated by a narrow seaway some 180 million years ago, and similarly the west coast of Africa and South America. It is quite an astonishing thought that it was once possible to walk from Africa to Antarctica many millions of years ago. There were no humans around to make that journey, but the modern-day examples of the Red Sea (Figure 3.14) and the East African Rift Valley which we have just discussed are fabulous. Moses and the Children of Israel would have had to scramble their way over a hot spreading centre on their flight from Egypt.
Figure 3.14 The Red Sea from space, looking north towards Suez and the Gulf of Aqaba
Convergent Plate Boundaries
Where oceanic crust dives beneath other oceanic crust or continental crust, this is called a convergent plate margin, and the subduction takes place along what is called a subduction zone. There is another special case where continental crust meets continental crust and we shall deal with that shortly, but the predominant mechanism of convergent plate margins is subduction.
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Remember that as new crust is created at the mid oceanic ridges there is a corresponding destruction of older crust elsewhere, otherwise there will be a need to invoke an expanding- earth hypothesis to deal with all the new crust being formed. As we now know, destruction of the oceanic crust takes place along subduction zones. These are also known as convergent plate margins.
There are three settings for convergent plate margins, namely • Oceanic -oceanic convergence • Continental -oceanic convergence • Continental-continental Convergence
Oceanic-Oceanic Plate Convergence
Where two oceanic plates collide, the cooler, denser oceanic lithosphere sinks beneath the warmer, less dense oceanic lithosphere. The subducting plate descends at angles ranging from 40 to 60 degrees from the horizontal. The convergent margin is characterised by earthquake activity that can extend to depths of up to 700 km. Hugo Benioff, whom we met in Chapter 1, plotted up the positions of earthquake epicentres (this data can be determined using seismographs), and found that they occurred along a plane that descended below the overriding plate. For reference see Figure 1.29 in Chapter 1.
Benioff concluded that the oceanic trenches were the locality of deep-seated faults which led to the earthquakes. Back then plate tectonic theory was still in its infancy, but once the mechanisms of sea floor spreading were understood, with a concomitant need to find a mechanism for the destruction of older oceanic material elsewhere, it was realised that these fault zones were subduction zones where crust was destroyed in a juddering descent back into the fiery underworld.
The position where one plate dives below the other is called a trench or foredeep. As the plate descends, temperatures increase. However, the cold, descending plate, 7 km thick, drags the temperature down in the zone immediately surrounding the plate and remains rigid enough to grind its way down to 700 km before it begins to melt.
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Eventually melting does takes place, assisted by the presence of water held in hydrous minerals within the subducting slab, and from water from the oceans above. The water actually lowers the melting point of the basalt by acting as a flux. The molten rock, called magma, then works its way towards the surface as diapers, to ultimately erupt as volcanoes. Japan’s Mt Fujiyama, Indonesia’s Mt’s Merapi, Bromo, Tambora and Krakatau are good examples. Krakatau and Tambora were two of the largest eruptions to have ever been recorded in the last 10 000 years, with the eruption of Tambora killing an estimated 71 000 people. Oceanic Plate Convergence is shown in Figure 3.15.
Figure 3.15 Oceanic Plate-Oceanic Plate Convergence.
Continental - Oceanic Convergence
Similarly to the convergence of oceanic plates, the denser oceanic plate is forced to descend below continental crust. Hydrous minerals in the descending slab once again provide the flux which reduces the melting point of the plate at depth leading to the formation of diapers which rise up through the crust to erupt at the surface as volcanoes to form a volcanic arc. Sediments from the continent, as well as deep sea sediments further offshore, accumulate in what is called an accretionary (sedimentary) wedge, which is then scraped from the oceanic
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plate where it dives beneath the continental crust. These sediments are rucked up and folded due to the bull-dozing effect of the over-riding plate. The mechanisms of Oceanic-Continental Plate Convergence are shown in Figure 3.16.
Figure 3.16 Oceanic-Continental Plate Convergence
The most famous example is the Andean system (3.17), where the Pacific plate is being subducted below the South American plate. The Andes have literally dozens of volcanoes which run along the western spine of the continent with Cotopaxi and Galeras being good examples of volcanoes. Mt St. Helens is another good example in the continental United Sates. It erupted in 1980 and killed 57 people and laid waste to thousands of square kilometres of forests and farmland. Geologists were besides themselves with delight, in spite of the death and destruction that took place, for they were able to monitor and film the entire eruption from beginning to end. The Aleutian Islands and the Kamchatka Peninsula are further examples of continental-oceanic convergence.
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Figure 3.17 Andean scenery
Volcanoes from a geohazard point of view will be covered in a future e-book.
Continental – Continental Convergence
Ongoing subduction of an oceanic plate with trailing continental crust below a continental margin will eventually see the consumption of the oceanic crust and the collision of the continental slabs. Continental material is buoyant and it is therefore impossible for one plate to subduct below the other, and so collision is inevitable with associated tectonic upheaval.
As the two continents approach, any sediment which may have accumulated on ocean floor is scraped off and rucked up by the advancing continents. This can include huge coral reefs such as to be found in modern examples of the Bahamas or the Great Barrier Reef. Some slight subduction may take place, but ultimately huge slices of material from one of the plates will be sheared off, thrusting up and over, folding and overturning like we saw earlier in our tablecloth example. The crust is thickened in these collision zones, extending to depths of 75 km in some instances.
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Figure 3.18 Continental-Continental Plate Convergence (image from www.soest.hawaii.edu)
At these depths, rocks are metamorphosed under immense heat and pressure. Some of the rock may melt and rise as granite batholiths within the roots of the newly formed mountain range. The two continents become welded together. These are known as mountain belts, or fold mountains. Figure 3.18 shows the components of a continental-continental collision zone – in this case the Himalayas which is a classic example of this kind of collision. India ploughed into Asia some 50 million years ago after a 130-million-year journey from its original position as part of Gondwana. Figure 3.19 is a generalised section through the Himalayas.
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Figure 3.19 A section through the Himalayas
The highest point on Earth, Mt Everest, is a result of this collision and India and Asia are still converging at around 5 cm per year. Nanga Parbat, one of the major peaks in Pakistan controlled Kashmir, holds the record of the fastest rising portion of crust on Earth, rising at a rate of 10 mm per year. The sea that once lay between India and Asia was called the Tethys, and sediments from this lost ocean are now preserved on land, along with some fabulous fossils including those of extinct whales. The summit of Mt Everest comprises ancient limestones which as we now know, are the remains of ancient coral reefs.
The Alps is another fine example of continental collision as Africa ploughed into Europe. Alpine geology is much better understood than Himalayan geology because it has been studied for longer, it is not as remote, high and inaccessible as the Himalayas, the pioneers of geology were European and finally the Alps has had several tunnels driven through it which has provided insights into the geology of the deep core of the mountain range - information which is not available to Himalayan geologists.
Some of the classic theories of geology were worked out in the Alps. A cross section through this mountain range is a beautiful thing; beautiful in that it provides a window into the deep
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structure of a tectonic region, and beautiful in that it provides an insight into the amount of work that went into deciphering the structure of these mountains. Those old hand-drafted sections seem to convey a certain permanence, a sense that everything has now been worked out and the matter settled.
However, there is one particular section which provided three generations of geologists with an interesting challenge. Naturalist Hans Conrad Escher von der Linth (1767–1823) had found older rocks on top of younger ones, which flew in the face of one of the basic tenets of geology – the Law of Superposition - which stated that younger rocks will always overlie older. Escher found Permian-aged Verrucano rocks overlying younger Cretaceous and Palaeogene mudstones and sandstones with the two formations separated by what we now know as the Glarus Thrust – a distinct line that could be traced out on the mountain side. Perhaps what drew attention to this particular locality was Martinsloch, which according to legend had been formed when local hero Martin had fought a troll, whose finger had punched through the ridge to form the loch or hole.
Figure 3.20 Verrucano sandstones and shales overlying younger limestones
Escher’s son Arnold also took an interest in geology, and became the first professor of geology at the ETH in Zürich. Following in his father’s footsteps, he mapped out the geology of the Glarus thrust and associated rocks in much greater detail and concluded that the feature was part of a huge folded nappe. If this was true then the rocks of the Verrucano would have had to have been transported 100 km northwards to their present position, with the movement
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having taken place along the Glarus thrust fault - the distinct line visible from the town of Elm. Figure 3.20 shows Verrucano sandstones overlying the younger limestones. The line is distinct.
This however seemed to be too fantastical an idea so he called in Roderick Murchison, a British expert on these matters, for assistance. Murchison, based on his experience in the Scottish Highlands, agreed that it was indeed a single thrust. However, Escher lost his nerve and took the conservative approach, invoking the idea of two nappes being thrust from opposite directions to explain the distribution of the rocks. We must remember that Escher junior did not have the luxury of plate tectonic theory to assist him in his deliberations, and we have already seen how supposedly crazy theories can be shot down by the scientific community.
Figure 3.21 Two sections through the Alps with the original interpretation (top) of the geology and the subsequent revised interpretation (bottom) based on nappe theory.
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In the original cross sections in Figure 3.21 (a) above, drawn through the Helvetic Nappes, we can see Escher’s interpretation of the geology. Escher’s successor, Albert Heim, first accepted Escher’s double nappe theory but as more evidence was collected, Heim eventually came around to the single-nappe theory – Figure 3.21 (b). Once this theory had been accepted, he and his fellow Swiss geologists recognised nappes and thrusts everywhere, and this new theory became the key to unravelling the complex geology of the Alps. The revised cross section shows the same surface outcrop but was now explained by the simple single-nappe model that Escher Senior had originally postulated, but which had also made him lose his nerve.
The collision of Africa with Europe thrust huge nappes up and over the entire fold belt until they came to rest in their current positions. If you go to Martinsloch in the Glarus region of Switzerland, you can see this great nappe exposed high on the mountain side. On March 12th and 13th, and September 30th and October 1st of each year, the rising sun shines through the ‘loch’ onto the Church of Elm in the valley below. Martinsloch and the Glarus Thrust is geology writ large – older Permian aged Verrucano rocks lying on top of the younger Jurassic and Cretaceous limestones.
Figure 3.22 Spectacular Himalayan scenery – a classic example of fold mountains
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I have tried to convey the shear wonder of it all in our Alpine example. The other grand example is the Himalayas (Figure 3.22) which are more than 8 km high, thrust up into the jet stream where ice, rain and snow lash the mountain tops, grinding them down and depositing the debris into the oceans. Rivers tumble from the high ground, and glaciers carve out spectacular U-shaped valleys that take our breath away when the ice retreats. For geologists of a certain type, metamorphic terranes such as the Himalayas and the Alps provide endless scope for exploration, adventure and scientific enquiry, often in exotic or deeply significant places. Mt Kailash, shown in Figure 3.23, is for example Hinduism’s most holy mountain and of course a geological paradise.
Figure 3.23 Chortens with Mt Kailash in the background, Himalayas
Folding, fracturing, thrusting, metamorphism, mineralogy and palaeontology are all topics of study in these tectonic regions. Geologists scramble up mountain sides in some of the most beautiful and spectacular landscapes on Earth, with some of the most fascinating cultures that only mountainous regions can develop. Prayers flags whip in keen mountain breezes, rivers run milky white, and mountain goats and yaks pick their way along mountain paths without a
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care and local heroes fight trolls and wolves in their high mountain redoubts. Who cannot be moved by these spectacular examples of the machinations of our planet?
Passive Plate Margins
Where it all began – in the Atlantic. If you look at a figure of the plates, you will notice that there are no subduction zones either side of the Atlantic. Okay, there is some activity in the Caribbean, but in general there are no active zones. These margins are called passive margins. Spreading at the mid-Atlantic ridge drives the plates apart, but there is no corresponding destruction of crust adjacent to the continental margins. Figure 3.24 shows the components of a Passive Plate Margin.
Figure 3.24 Passive Plate Margin
Conservative Plate Margins
These are also called transform faults. Canadian geologist John Tuzo Wilson, whom we met earlier, first recognised them in the Pacific whilst looking for a mechanism to accommodate differential rates of ocean floor spreading. Transform faults occur when two blocks slide past each other, with the movement being horizontal, although sometimes with the faults going at different speeds relative to each other depending on which side of the spreading centre they find themselves. Things can get a little complicated when trying to describe the mechanism in words, so Figure 3.25 will assist in explaining matters.
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Figure 3.25 Conservation Plate Margins along a transform fault.
No crust is therefore created or destroyed at a Conservative Plate Margin. However, they are fundamentally important in allowing different parts of the crust to slide past one another at different rates, which accommodates the different spreading rates of the ocean floor. It is like a differential on a car – when going around a corner the kerbside wheels have to travel a shorter distance than the roadside wheels, and this needs to be accommodated within the differential mechanism of the transmission system.
Figure 3.26 shows transform faults from the Marie Tharp – National Geographic map of the ocean floors. There are literally thousands of these faults but we have highlighted a few from the Atlantic for illustrative purposes.
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Transform Faults
Figure 3.26 Transform faults (in red) in the Atlantic.
Conclusion
This has been quite a journey. We have travelled with the likes of Abraham Ortelius, Roberto Mantovani, Arthur Holmes, Harry Hess and of course Alfred Wegener. Rusty signposts lay along the route. It has been a pilgrimage of a sort – a pilgrimage to sacred places on Planet Earth and sacred places in the annals of the Earth Sciences. Earth history is writ large in the rocks and the geologists and geophysicists have done their work in deciphering the secrets of our planet. We take it all for granted now, and we only read about the success stories, but without a doubt there were many wrong turns and dead ends which no one wants to remember. Heartbreak there was without a doubt, unacknowledged heroes aplenty, as well as some casualties along the way.
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A revamp of a familiar Figure
I hope that I have injected some life into this story of plate tectonics. There are things that you don’t need to know for the examinations, but which I think are important nonetheless. The stuff you need to know is listed in the text books and I have also prepared a summary below of the important points which you need to cover in the examinations. My goal was to create some kind of excitement around the subject – imagine Alex du Toit getting on a ship and crossing the Atlantic to meet his counterparts in South America. Imagine setting out on a voyage into the beautiful blue Pacific to take geophysical readings of the ocean floor. Imagine climbing Alpine slopes to examine the geology of some thrust fault that lay 400 metres up a mountain side. Imagine descending in the submersible Alvin to see the black smokers on a mid-oceanic ridge. If you can imagine it, you can do it, for there is absolutely nothing to stop you from signing up for a course in geology or oceanography and going on adventures of your own.
But first you have to pass these exams on this early part of your journey, so sharpen that pencil and come exploring with me.
The End
Rock and Sky